Decking – The Most Expensive Part of a Flat Roof?

The large sheets of decking underneath most flat roofs may need to be replaced if you’re installing a new roof covering.

Recent research has shown that roofing companies charge between £50 and £60 to replace each sheet with a new OSB ply board.

A typical single garage has up to 6 sheets. 6x £60 = £360 and that is just the decking!

But do you need to replace the existing boards or can they be reused?

Here is our guide:

If you want a fibreglass roof then yes, in most cases, you will also need to replace the decking boards.

If you have chipboard decking under your existing roof then yes, in all cases it should be replaced as chipboard is unsafe and when wet, will not support the weight of someone walking on the roof.

If you have plyboard under your existing felt, then each board will need to inspected for rot or distortion and replaced as required. Plyboards are usually long lasting and durable so shouldn’t need replacing unless subject to water for a period of time.

If you have close-boarded decking (a particularly strong type of board) under the felt, then it shouldn’t need replacing unless you want a fibreglass roof covering.

Here is a pro tip from an experienced roofing contractor with over 20 years experience:

check the roof for any dips, bowing or pools of water. This is usually the result of a failed decking board and it will need to be replaced. If it isn’t then rainwater will pool on the new roof and potentially shorten its lifespan.

What Type of Flat Roof Covering is Best?

Below is a table exploring our personal opinion of the most popular types of roof covering (bitumen, asphalt, rubber and fibreglass).

We suggest you have a think about what is most important to you and how this fits in with your budget.

Product:

Price:

Life

Looks

Flex

Strength

Bitumen Felt

9/10

12-20 yrs

5/10

7/10

7/10

Fibreglass

6/10

25-40 yrs

10/10

4/10

9/10

Asphalt

8/10

20-30 yrs

8/10

3/10

9/10

Rubber: 1 Piece

8/10

25-40 yrs

7/10

10/10

8/10

Rubber: Jointed

7/10

25-40 yrs

3/10

9/10

8/10

The visual appeal of a flat roof that is overlooked by several prominent windows in the house is probably more important that a dormer flat roof located so high up that you can’t see it from any window.

For those on a low budget, a bitumen torched-on felt system is usually advised. These products have been around for decades and there’s plenty of competition that keeps the prices down.

Fibreglass and rubber roof systems last the longest but are at the upper end of the price range.

Pros and Cons of Each

Below is a guide to each type of flat roof covering:

Fibreglass

This material has been used for decades and is often seen on boats. It was first used on flat roofs several decades ago but got an undeserved bad reputation due to poor workmanship and techniques that have now been improved with updated standards.

Positives:

it’s visually appealing

tough and unlikely to crack from impacts such as dropped tools

easy to patch repair

easy to clean with a mop

insects, moss and mould won’t damage it

no naked flame used during installation

Negatives:

should be installed by a specialist with experience of this material

not as reliable on large flat roofs that experience flexing of the roof timbers

price – at the top end of the price range

standard fibreglass is slippery, especially when wet

Torch-on Bitumen Felts

This type of felt, also known as a built-up or layered felt system, is manufactured in rolls and has a tar-like material called bitumen on the back which is heated with a gas torch as the material is unrolled over the roof.

The felt is built up into layers. A 3 layer system is typically used over a habitable part of the home, while on garages and sheds it’s not uncommon to see a 2-layer application.

Positives:

affordable and is in the middle of the price range

plenty of roofing businesses do this type of work so there’s lots of competition

can be patch repaired if damaged by falling tree branches or similar objects

popular for large roof areas

can be joined to most materials if you share a flat roof with a neighbour

Negatives:

looks patchy as each roll is joined to the next with a visible line of bitumen (tar)

doesn’t last as long as modern alternatives as it’s affected by sunlight

can’t walk on it during hot weather as the material is much softer and may stick to the underside of shoes

requires extensive use of gas blow torch during installation which means increased fire risk and health and safety considerations

Rubber Membranes

These first hit the markets about 20 years ago and have become very popular due to their longevity. There are essentially two types of rubber membranes in use today and they are slightly different.

There are essentially two types of rubber membranes in use today and they are slightly different.

The one piece system has no joints and is made to measure. This is typically used on smaller roofs such as single garages and dormers.

The most common type of rubber flat roof comes in rolls and each joint is sealed with a chemical glue that bonds the material together.

Both systems come with edging trims and accessories to finish off the roof neatly.

Positives:

a long lasting product that isn’t easily affected by sunlight or insects

impact resistant up to a point

no gas torch used in the installation

were very expensive but prices have come down slightly due to competition

the one-piece rubber membrane is attractive as it doesn’t have any joints

it flexes with the roof and shouldn’t crack or split

easy to clean with a mop

lightweight

Negatives:

the price – still at the top end of the range

the one-piece version has been known to pull away from any vertical walls due to thermal movement, this is worse if it hasn’t been correctly fitted. Extra care and attention should be made to large areas as they will experience more thermal movement

the jointed version isn’t visually appealing and can look plasticky and patchy

there are plenty of cheap, inferior and low-quality one-piece variants on the market that target DIY installations. These typically have poor quality trims, adhesives and have a tendency to form ripples and creases.

Mastic Asphalt

This material was once one of the most popular flat roof materials in the UK and has been in use for hundreds of years. However, poor workmanship by a minority and the emergence of new products that are quicker and easier to work with has reduced its popularity.

Asphalt is often used in waterproof tanking and damp proofing. It is one of the best waterproof materials in use but does have some limitations.

Positives:

looks appealing with a consistent smooth grey finish

has a very tough and durable surface that won’t break when objects are dropped onto it, can be walked on all year round

should never need a complete replacement, just an extra layer added to the top when it comes to the end of its life

Negatives:

asphalt is very heavy and some flimsy roof structures are not suitable for it

considered by some to be an old and outdated way to waterproof a flat roof. Fewer and fewer companies now offer this service as it’s more difficult to lay than a roll of felt.

Insulation – A Quick Guide

Since 2006 all flat roof replacement projects around the home must now meet minimum insulation thresholds.

So, if your flat roof is over a habitable part of the house, you’ll need to ensure that adequate insulation is also included.

That could mean extra expense, although it depends on how much (or little) insulation you currently have in this part of the roof.

The good news is that insulation is not too expensive and there are tons of options available. Your house will also be more efficient so you’ll get some of your money back over time due to reduced energy bills.

Insulation is not required for garage flat roofs or sheds.

Neither do you need to meet this requirement if you are laying a new roof covering over the top of an existing one.

The insulation rules only apply to flat roof replacements, not refurbishments/repairs.

Below is a decking product that contains a sturdy ply deck, a thermal insulating layer and a waterproof layer to keep condensation out:

Celotex insulated flat roof decking sheet

These boards are 1200mm wide and 2400mm long and cost around £55-60 inc vat (exc fitting fee).

They are used instead of traditional decking sheets, so if you intend to replace your old sheets, just go for this product instead, they are more expensive but they’ll meet current building regulations.

A typical small dormer will require one sheet.

A small extension will need 3-4 sheets and larger extension 5+ sheets.

There’s no need to use these on separate garages and sheds as they aren’t usually insulated or heated anyway.

What About Prices?

We have created a helpful guide to flat roof prices that you can see below.

This guide includes example prices for torch-on felt, rubber membranes and fibreglass: